Why does the rear pilot need better visibility during landing?
The rear pilot needs better visibility compared to not having a periscope.
It isn't "better visibility" than the front cockpit.
For trainer/instructor purposes.
That said, in future you can probably just search an answer to this question online.
Why is this relevant in the PLAAF section?
I think there's no general "ask anything thread" about general military aviation matters so they've probably perceived this "ask anything thread" as being for that purpose rather than being PLA specific.
We're a PLA specific forum, but we will field some general military questions now and then.